Assessment of exposure to drug use and psychosocial stress is complicated by the fact that each is often transient and difficult to recall accurately. Assessment of their causal connections with one another, and of their genetic and environmental determinants, is complicated by the complexity of the causal connections and by the elusive nature of what constitutes the environment. In the proposed study, we will assess drug use and psychosocial stress in near-real time through Ecological Momentary Assessment (EMA), in which participants use handheld electronic diaries to record events as they occur and to report recent or ongoing events in response to randomly timed prompts throughout the day. We will also maintain real-time records of where the reported events occurred by having participants carry Global Positioning System (GPS) devices to track their whereabouts with a likely spatial resolution of several meters. Broadening the definition of environmental factors, we will evaluate neighborhood-level exposures to drugs and stress measured in terms of specially developed indices, the Drug Environment Index (DEI) and the Neighborhood Psychosocial Hazards index (NPH), each based on objective statistical data available from public sources and independent of self-report. For comparison, drug exposure will also be assessed through retrospective audio-computer assisted structure interview (ACASI) and from biological specimens (hair and sweat), and stress will be assessed through ACASI and physiological measures (heart rate and allostatic load). The project represents a collaboration between the NIDA IRP (where we are already using EMA with polydrug-dependent outpatients) and Johns Hopkins ALIVE Study investigators (following a well-characterized community cohort of drug abusers, most of whom are not in treatment). After developmental work at NIDA linked to field trials in ALIVE, the result is expected to be a set of field-deployable, state-of-the-art tools indispensable to future studies of gene-environment interactions affecting drug use and stress.